This invention relates to the field of speed controls for rotary machines and, particularly, to a speed control which utilizes plugging to assure stability of a rotary machine at a selectable synchronous speed over a wide range of loads and line voltages. Additionally, this invention relates to a control system for high speed rotary machines with three or more phases, and, particularly, to a control system which responds to voltage induced across unpowered stator windings by a rotating rotor magnetic field to control application of voltage pulses to the stator windings.
Rotary machines have generally been classified as either DC or AC machines. Both types of machines include means for generating at least two different magnetic fields, one field being produced in a rotor and the other field being produced in a stator. When these fields are not aligned with each other, a torque is established which brings about a rotary movement of the rotor. As the rotor turns, the orientation of the rotor field with respect to the stator field changes. As the field orientation changes, the magnitude of the torque on the rotor also changes so that the machine must include some means for reorienting one magnetic field with respect to the other so as to maintain the torque on the rotor near a maximum in order to sustain efficient operation.
In DC machines, the rotor armature normally has a plurality of windings wound thereon which are connected to an external power source through brushes and a mechanical commutator. The commutator causes voltage to be applied selectively to the armature windings so that the magnetic field produced in the rotor on the average will be aligned at a 90.degree. angle to the stator magnetic field produced in fixed stator windings or by a permanent magnet stator. Because the angle between the armature field and the stator field for a DC machine is on the average 90.degree., the DC machine is generally efficient, and torque on the rotor is maximum. However, the DC machine does have a drawback over most AC machines. When the DC machine is loaded by an external device connected to the rotor, the angular velocity of the rotor decreases in a somewhat linear fashion as the load increases. Consequently, DC machinery is not suitable for use in applications where constant speed is required and the load and/or line voltage varies unless some form of auxiliary speed controller is utilized.
AC synchronous machines, on the other hand, are designed to utilize an alternating current power source to power and synchronize the machine. The fixed frequency of the AC power source is automatically operative to cause rotation at a fixed angular velocity. When the synchronous machine is more heavily loaded, the machine is operative to cause the torque angle, the angle between the rotor field and the stator field, to come ever closer to a 90.degree. angle. In this manner, the torque on the rotor increases as the load on the machine increases. Consequently, an AC synchronous machine is able to maintain a constant speed even as the load varies. The problem with AC synchronous machines is that such machines do not operate at optimum efficiency for all loads. Indeed, the AC synchronous machine is most efficient when it is heavily loaded because the torque angle approaches 90.degree.. When the AC synchronous machine is less heavily loaded, the torque angle is less than 90.degree., thus reducing the machine efficiency.
In addition, AC synchronous machines do not have the ability to readily adjust speed and are well known to have a tendency to be unstable. The torque angle of the synchronous machine is determined by the load. If the load changes, the torque angle will change to accommodate the new load. As the AC synchronous machine changes toward a new torque angle, it will overshoot and then come back toward the new torque angle so as to oscillate into the new torque angle. An AC synchronous machine, however, depending on the amount and rate of load change, can oscillate so badly that it will drop out of synchronism unless means such as damper windings are provided for reducing oscillation.
The rotary machine which is described in Ser. No. 561,537 filed Mar. 24, 1975 incorporates the desirable features of both the DC commutator machine and the AC synchronous machine. Specifically, the rotary machine operates efficiently as a DC commutator machine and also operates at a selectable synchronous speed over a wide range of loads and/or line voltages as an AC synchronous machine. Moreover, the rotary machine does not have the drawbacks of other DC brushless motors or transducerless machines since operation of the rotary machine control system is substantially independent of machine size.
Although the rotary machine which is described in aforementioned Ser. No. 561,537, in contradistinction to an AC synchronous machine, operates as a DC commutator machine because it operates at a constant torque angle, due to the fact that the rotary machine is preferably maintained at a selectable speed if the load changes, it has a tendency to become unstable as in the case of an AC synchronous machine. That is, if the load changes, the voltage pulse amplitude and/or width will change to accommodate the new load. If the load changes rapidly by a substantial amount, however, the voltage pulse amplitude and/or width that is necessary to maintain the rotary machine at the selectable speed can be overshot, then undershot, and then overshot again, etc. so that the rotary machine oscillates and possibly drops out of synchronism.
In the preferred form of the rotary machine in aforementioned Ser. No. 561,537, the control system responds to voltage induced across unpowered stator windings by the rotating rotor magnetic field to control the application of voltage pulses to the stator windings. When such a machine with three or more phases is operated at high speed, however, transformer action among the stator windings which occurs at times when voltage pulses are applied to and removed from stator windings affects the voltage across unpowered stator windings. This in turn can cause improper control of the application of voltage pulses to the stator windings.